1. Field of the Invention
This invention relates to an objective lens system of an optical information recording/reproducing apparatus in which a semiconductor laser is used as a light source.
2. Description of the Prior Art
An optical system of an optical information recording/reproducing apparatus, such as optical disk apparatus, etc. includes a light source portion having a semiconductor laser, and an objective lens system converging (focusing) the luminous flux emitted from the light source portion onto an optical disk, and a signal detecting optical system which is adapted to read information recorded in the disk and error signals by reflected light from the optical disk.
Also, the objective lens system is mounted on an actuator which is designed such that the objective lens can be finely moved at least in the optical axis direction thereof (focusing servo) so that an out of focus condition caused by warping of the disk, etc. can be corrected.
FIG. 40 shows an prior art objective lens system constructed of four pieces of lens glass. Concrete numerical examples are as shown in Table 1. In the table, the reference character NA denotes the numerical aperture, f denotes a focal length of the objective lens system at a wavelength of 780 nm, .omega. denotes a half field angle, wd denotes a working distance between last surface of the objective lens system and the optical disk OD, r denotes the radius of curvature of a surface, d denotes a lens thickness or a spatial distance, n.sub.780 denotes a refractive index at a wavelength of 780 nm of a lens, and .nu..sub.780 denotes a dispersion of a lens in the vicinity of wavelength 780 nm.
TABLE 1 ______________________________________ NA = 0.55 f = 3.29 .omega. = 1.7.degree. wd = 1.487 surface No. r d n.sub.780 .upsilon..sub.780 ______________________________________ 1 45.570 1.330 1.61139 1451 2 -3.042 0.700 1.82484 554 3 -26.608 0.080 4 5.310 1.200 1.61139 1454 5 -12.181 0.080 6 2.600 1.020 1.76204 1013 7 4.875 ______________________________________
Various aberrations of this objective lens system are shown in FIG. 41 and the wave aberrations are shown in FIG. 42.
By the way, an emitting light wavelength of the semiconductor laser used as a light source is shifted by changing of output power and/or temperature. Because of the foregoing reason, when the chromatic aberration of the lens system is not yet corrected, the position of a light converging point is varied by shifting the wavelength. But an out of focus condition due to a comparatively gentle change of a wavelength caused by change of temperature or the like is automatically corrected by the afore-mentioned focusing servo when the collimator lens is corrected in chromatic aberration and change of temperature.
However, at the time when data is written, an oscillation wavelength of the semiconductor laser is instantaneously shifted by several nm between a region where the temperature is increased and a region where the temperature is not increased. And the out of focus condition caused by such radical shift cannot be corrected by the above-mentioned focusing servo.
When the light converging position is not coincident with the recording surface of the disk, there is a high possibility that incorrect writing (i.e. recording) and reading are effected.
Therefore, especially when writing is effected, correction of the chromatic aberration of the objective lens system is important.
An optical system in which the converging lens itself is corrected in chromatic aberration is disclosed in, for example, Japanese Patent Early Laid-open Publication No. Sho 63-10118, Japanese Patent Early Laid-open Publication No. Sho 60-232519 and Japanese Patent Early Laid-open Publication No. Sho 58-72114.
The lens of the Japanese Patent Early Laid-open Publication No. Sho 63-10118 is of a three piece structure including an aspherical lens, while the lenses of the Japanese Patent Early Laid-open Publication No. Sho 60-232519 and the Japanese Patent Early Laid-open Publication No. Sho 58-72114 are of a four piece structure of lens glass.
However, the lenses which are disclosed in above publications are short in working distance. Therefor, there are such problems, such as that the range in which the objective lens system is movable, is too narrow.
Also, Japanese Patent Early Laid-open Publication No. Sho 62-269922 discloses an optical system for correcting the chromatic aberration of an objective lens by excessively correcting the chromatic aberration of a collimator lens. With this construction, it is necessary to excessively correct even a focusing error detecting optical system because otherwise an out of focus condition is generated, caused by a focusing servo.
However, the correcting amount of the chromatic aberration of the focusing error detecting optical system is proportional to a second raised power of the ratio M between a focal length of a condenser lens of this optical system, and a focal length of the objective lens. Therefore, in an ordinary optical disk apparatus, taking a value of about M=10 in view of the size of a light receiving element, it is difficult to design a system where the condenser lens has a sufficient correcting amount of chromatic aberration.